In order to ensure that two-stroke engines have a high power capacity at high speeds, a high volumetric efficiency is required and the charge losses must be minimized. This can be accomplished by an early and therefore higher opening of the exhaust passage into the cylinder. The adjustment of the exhaust port, to obtain maximum power capacity of the engine at high speeds involves, in the medium speed range, not only an appreciable decrease of the useful stroke, but also a large increase of the charge losses. As a result, the torque decreases and the specific fuel consumption increases greatly. A higher torque in conjunction with a lower fuel consumption can be obtained, at lower engine speeds, only if the opening of the exhaust port happens later in the down stroke of the piston which means that the exhaust port must be at a lower position than it is at high engine speeds.
For this purpose it is known to provide, in the exhaust port, a valve which is movable between a full flow position and a flow restricting position. When in the flow restricting position, the end of the valve is substantially flush with the peripheral surface of the cylinder bore. In this flow restricting position, the exhaust port is effectively lowered in relation to the down stroke of the piston. The valve is adjustable to vary the relative height of the exhaust port as is required by the given operating conditions of the engine.
To control the position of the valve, various types of actuators have been provided. One such type of actuator is a pneumatic actuator having a diaphragm such as the one described in United States Patent Publication No. 2009/0088942 A1, published Apr. 2, 2009, the entirety of which is incorporated herein by reference. Although, actuators of this type adequately control the position of the valve, they have a substantial amount of parts which increases the complexity, assembly time and cost of the valve assembly.
Furthermore, valves of the type described above need to be lubricated. This typically requires that a number of fluid lines be connected to the valve assembly in order to supply the required lubricant. This further increases the complexity, assembly time and cost of the valve assembly.
Therefore there is a need for a valve assembly having an actuator with a reduced number of parts.
There is also a need for a system for supplying lubricant to the valve assembly.